When hydrosphere organisms are bred in a retention tank such as a water tank, organic matter derived from excrement thereof and the like generates toxic ammonia, nitric acid, and the like. Accordingly, the excrement and the like have been conventionally removed by using a filtration device as well as periodically performing a water replacement process of replacing breeding water in the retention tank with fresh water and a replacement process of replacing sand in the retention tank with new sand. However, an external filtration device of a type provided above the retention tank or the like requires high costs for installation and maintenance management. These costs will increase if the retention tank is large in scale. Furthermore, the processes of replacing water and sand require time and effort and may damage hydrosphere organisms more than a little.
A breeding system not including such an external filtration device mentioned above is typically exemplified by a breeding system of Jaubert's Monaco System disclosed in Patent Document 1. This breeding system includes a water permeable plate member providing a raised bottom, a sand layer provided on the plate member, an aerobic layer provided on the sand layer, having an aerobic environment, and receiving light, and a facultative anaerobic layer provided under the sand layer, having an anaerobic environment, and receiving no light. Aerobic bacteria inhabit the aerobic layer whereas facultative anaerobic bacteria inhabit the facultative anaerobic layer. The aerobic bacteria and the facultative anaerobic bacteria decompose toxic ammonia, nitrous acid, nitric acid, and the like generated from organic matter to exert a filtering function. The breeding system of Jaubert's Monaco System typically does not require the external filtration device mentioned above.